The present invention relates to feed systems to deliver comminuted cellulosic fibrous materials, such as wood chips, to a continuous digester treatment vessel and, particularly, relates to feeding these materials to a high pressure transfer device that converts a low pressure slurry of the material to a high pressure slurry of the material that is transported to the digester treatment vessel.
In the pulping of comminuted cellulosic fibrous material (generically referred to herein as “wood chips” or just “chips”) in a continuous digester, the wood chips are treated to remove entrapped air and to impregnate the chips with cooking liquor while raising the pressure and temperature of the material to, for example, 150 degrees Celsius (150° C.) and 10 bar gauge (g). Typically, the chips are steamed to purge air and increase the temperature of the chips, impregnated with heated cooking liquor, pressurized and transported as a slurry to the digester.
A conventional chip feeder assembly typically includes a chip bin, a low pressure feeder, a steaming vessel, a vertical chip tube and a high pressure feeder to purge air from, heat and pressurize the chips. Examples of conventional high pressure feeder assemblies are disclosed in U.S. Pat. No. 5,968,314, which shows a chip feeding system for a digester having a vertical chip bin, a horizontal chip steaming vessel, a vertical chip tube (also referred to as a chip chute), and a high pressure feeder.
Conventionally, the chips in the chip bin are relatively dry are slurried with liquor downstream of the chip bin. However, liquor has been added to the chip bin to slurry the chips to facilitate transport of the chips from the chip bin to the steaming vessel and the vertical chip tube.
Steam has also been added to the chips in the chip bin or in a steaming vessel. The chips may also be steamed in a steaming vessel downstream of the chip bin. At the discharge of the steaming vessel or in chip conveyor, liquor has been added to the chips to slurry the chips to facilitate transfer of the chips.
The slurry has been moved by mechanical conveyors, e.g., horizontal tubes with screws and augers, to the vertical chip tube. The screws and augers in the mechanical conveyors are driven by motors that require energy. These moving mechanical components, e.g., screws and augers, are costly in terms of acquisition costs and operating costs. There is a long felt need to reduce acquisition costs, maintenance costs and energy costs in chip feed systems.
The hydraulic pressure of the slurry in the vertical chip tube assists in feeding the chips to the high pressure transfer device. As the chip slurry enters the top of the vertical chip tube, the slurry fills the tube and applies a hydraulic pressure to the chips at the bottom of the tube. The chip tube has a height to ensure that the mass of the chip slurry held in the tube forms a sufficient hydraulic pressure at the bottom discharge end of the tube to feed the chips to the high pressure transfer device, such as a high pressure feeder.
Without sufficient the hydraulic pressure, the suction applied by the high pressure feeder to the incoming chip slurry may form gas bubbles in the slurry entering the feeder. Once gas becomes entrained in the slurry, the slurry becomes partially compressible and more difficult to pressurize. Gas in the slurry can reduce the efficiency of the high pressure feeder. In some circumstances, gas caused by lack of hydraulic pressure can block the flow of the chip slurry into the high pressure feeder.
Vertical chip tubes have conventionally provided the needed hydraulic pressure to a chip slurry. A vertical chip tube may be 15 feet to 30 feet (5 meters to 10 meters) in height. The height of the chip tube substantially increases the overall height of the chip feed system and requires the chip bin to be at a relatively high elevation above the chip tube. The supporting structures needed for the chip bin and other elevated portions of the chip feed system may be substantial. For example, the chip bin may be at an elevation of 115 feet (35 meters). There is a long felt need to reduce the height of the chip feed system to minimize the structure necessary for the system, and reduce construction and maintenance costs of chip feed systems.